This application proposes to characterize color appearance and to investigate how earlier visual mechanisms contribute to the production of color perception by the use of psychophysical methods. Following the initial stage of color perception in which photons are absorbed by three types of cones, the second stage processes visual information with one luminance and two chromatic mechanisms. The chromatic mechanisms are constructed by opposing inputs from different types of cones. However, they cannot explain our color percepts such as loci of unique hues and there has been evidence of higher-order color mechanisms to reach our color percepts. Unique hues are four primary color percepts which are pure and do not contain a tint of any other hues and play an important role in constructing a color appearance model. A better understanding of human color perception is essential to the development of artificial retinae and to the accurate assessment of congenital and acquired color vision defects. The specific goals are to measure unique hue loci on the equiluminant plane with various saturations that are very similar to the kinds of color stimuli we encounter in real life, to determine whether any earlier visual mechanisms, such as a Rayleigh match and orientation and relative sensitivity of cone-excitation axes, can account for individual differences in unique hues, and to determine the effects of contrast adaptation on unique hues in humans with normal color vision. To achieve these goals, human subjects will be asked to observe visual stimuli presented on the color monitor and to judge as to the presence or the appearance of those stimuli.